Frequency comb reaches extreme ultraviolet

Physicists in the US have created an optical frequency comb that operates in the extreme ultraviolet (XUV). Touted as the first practical comb to work in this region of the spectrum, the device could be used to look for tiny variations in the fine-structure constant and other physical constants that could point to new physics. An XUV comb could also be used to create better atomic clocks and new techniques for atomic spectroscopy.

Frequency combs are created with an ultrafast mode-locked laser, in which pulses of light bounce back and forth in an optical cavity. The frequency spectrum of the resulting train of pulses from such a laser is a series of very sharp peaks that are evenly spaced in frequency, like the teeth on a comb.

When one comb "tooth" is set to a standard frequency – such as that generated by an atomic clock – the absolute frequency of another light source can be measured to great accuracy by comparing it with the other teeth on the comb. The device therefore offers researchers a way of making very accurate spectroscopic measurements of atoms and molecules, and also a way of comparing atomic clocks.

Current combs operate at optical frequencies, and physicists have struggled to extend them into the ultraviolet and beyond. One promising path is a process called high-harmonic generation (HHG), whereby an intense laser ionizes atoms in a gas and then accelerates the electrons causing them to radiate high-frequency photons. HHG has already been used to create pulses of XUV light, but not trains of pulses that are of high enough quality to create a practical XUV comb.

A fine comb

One difficulty in making a practical XUV comb is ensuring that successive pulses have a high degree of phase coherence over time periods as long as seconds. Another challenge is making the pulses intense enough so that the comb can be used to perform atomic spectroscopy experiments. Now, however, Jun Ye and colleagues at the Joint Institute for Laboratory Astrophysics (JILA) in Boulder, Colorado, are the first to demonstrate a technique that addresses both of these problems.

The technique uses a high-power laser to create an intense infrared comb within an optical cavity. The cavity is then filled with xenon gas, which provides the medium for HHG, whereby the intense infrared pulses create pulses of XUV light. These XUV pulses bounce back and forth in the cavity to create a second comb. According to Ye, much of the glory for the demonstration should go to Ingmar Hartl and colleagues at the Michigan-based firm IMRA America, who designed and supplied the high-power laser.

Krypton factor

The comb was also operated using krypton as the HHG gas. In both cases, the team was able to create combs of light in the 40–120 nm wavelength range, which corresponds to XUV light. To demonstrate the comb, Ye and colleagues used it to study specific atomic transitions in argon and neon at wavelengths of 82 nm and 63 nm, respectively. In both cases they showed that light from a single tooth of the comb was intense enough to resolve the transitions. Patrick Gill of the UK's National Physical Laboratory described the work as "a good example of using the comb mode of HHG to do single-photon spectroscopy in the XUV".

Ye told physicsworld.com that the comb opens the door to a wide range of new measurements, including tests of single- and two-body quantum theory in atom-like systems. The combs could also be used in next-generation "nuclear clocks", which are based on nuclear transitions and "tick" at higher frequencies than atomic clocks. Other important applications could be laboratory and astrophysical measurements of variations of fundamental constants such as the fine-structure constant – which could point to physics beyond the Standard Model.

Ted Hänsch at LMU Munich described the work as "an important milestone on the path towards routine use of XUV frequency combs for spectroscopy". Hänsch – who shared the 2005 Nobel Prize for Physics for the invention of the frequency comb – told physicsworld.com "I am optimistic that frequency-comb techniques can be pushed to shorter wavelengths, but the required mutual phase coherence of successive pulses will make it rather challenging to reach the X-ray regime."

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4 comments

Combs cannot be clocks!

These combs of higher and higher frequency, hence , energy, will be a great help for the atomic spectoscopic work. However, they cannot repalce the atomic clocks, where the atomic transition frequecy remains unchanged over time, because one cannot run a laser without end in a stable mode. Moreover, it is not clear at all as to how these combs can be used to look for the time/ energy (?)variation of the fine-srtucture constant. Of course, we know as to how this fundamental constant varies with energy.

running lasers without end...

These combs of higher and higher frequency, hence , energy, will be a great help for the atomic spectoscopic work. However, they cannot replace the atomic clocks, where the atomic transition frequecy remains unchanged over time, because one cannot run a laser without end in a stable mode. Moreover, it is not clear at all as to how these combs can be used to look for the time/ energy (?)variation of the fine-srtucture constant. Of course, we know as to how this fundamental constant varies with energy.

Those combs need to tick too, "in tune" with multi-phased modulated output signals of the beam. Pulsed mode frequency train need rest period between each bunch of train pulses-enough time to cause overall rest periods to effect a considerable decrease in heat radiations from the lasing cavity(talking 'bout running lasers without end)in each second of wave propagation of the laser heads.Needs the application of the "nano-cones"- the ones used to convert infrared emissions to ultraviolet wave-lenghts. In this way,thus, reducing unwanted infrared wavelenghts that would practically reduce heat emissions and increased efficiency of the output transmission.This is expected too when the converted infrared signals participate in the main signal output transmission of the laser beam. It is suggested to test "router mode" action in the resonating cavity section of the laser. However, this setup requires multiple UV laser sources or even multiple resonating cavities compacted in a single laser head. At least , each laser source or the resonating cavity will have sufficient "cool down effect" when its off for a matter of milliseconds. The combs too should have "resonating processes" between each tooth. Each single tooth can be designed to emanate from each uv source of the system as partial component of the "router mode" action of the lasing process. The point here in the comments as a whole is a suggestion proposing for a setup that employs both heat conversion by nano cones and router-mode action by multi-sourced uv resonators.My concern in the success of these tests is on the application of the system for water splitters in water fuelled ICE's.

Frequency comb irrelevant comments again.

Once again the poster has struck, planting a pointer to a website that probably inserts malware into your computer. The poster(Victor Elias Espinoza Guedez Venezuelais), is making some bizarre claim about the composition of the universe. This is just silly noise and detracts from the pleasure of reading the results of physics research.Now the part about creating these higher energy, higher frequency combs is quite something indeed. I look forward to hearing what others are doing with this new technique.

These combs of higher and higher frequency, hence , energy, will be a great help for the atomic spectoscopic work. However, they cannot replace the atomic clocks, where the atomic transition frequecy remains unchanged over time, because one cannot run a laser without end in a stable mode. Moreover, it is not clear at all as to how these combs can be used to look for the time/ energy (?)variation of the fine-srtucture constant. Of course, we know as to how this fundamental constant varies with energy.

Those combs need to tick too, "in tune" with multi-phased modulated output signals of the beam. Pulsed mode frequency train need rest period between each bunch of train pulses-enough time to cause overall rest periods to effect a considerable decrease in heat radiations from the lasing cavity(talking 'bout running lasers without end)in each second of wave propagation of the laser heads.Needs the application of the "nano-cones"- the ones used to convert infrared emissions to ultraviolet wave-lenghts. In this way,thus, reducing unwanted infrared wavelenghts that would practically reduce heat emissions and increased efficiency of the output transmission.This is expected too when the converted infrared signals participate in the main signal output transmission of the laser beam. It is suggested to test "router mode" action in the resonating cavity section of the laser. However, this setup requires multiple UV laser sources or even multiple resonating cavities compacted in a single laser head. At least , each laser source or the resonating cavity will have sufficient "cool down effect" when its off for a matter of milliseconds. The combs too should have "resonating processes" between each tooth. Each single tooth can be designed to emanate from each uv source of the system as partial component of the "router mode" action of the lasing process. The point here in the comments as a whole is a suggestion proposing for a setup that employs both heat conversion by nano cones and router-mode action by multi-sourced uv resonators.My concern in the success of these tests is on the application of the system for water splitters in water fuelled ICE's.